1. Field of the Invention
The present invention relates to a ballast for a discharge lamp that is supplied with a phase-controlled AC voltage to light and dim a discharge lamp, in particular, a fluorescent lamp.
2. Description of the Related Art
Compared with an incandescent lamp, a fluorescent lamp has an advantageous feature of high efficiency and long life, so that it has been widely used, for example, in household lighting fixtures. In particular, the requirement to save energy and resources increases the demand for a bulb-shaped fluorescent lamp, in which a fluorescent lamp is integrated with a high frequency inverter, because the lamp can be inserted in an incandescent-lamp socket without modifying the socket.
In recent years, with the growing need for dimming a bulb-shaped fluorescent lamp like an incandescent lamp, a dimmable bulb-shaped fluorescent lamp has been under development. In the case of an incandescent lamp, a dimmer is used generally to supply a phase-controlled AC voltage for dimming. Therefore, to achieve the dimming of a bulb-shaped fluorescent lamp, it is necessary for a ballast circuit to be supplied with a phase-changed AC voltage so that the fluorescent lamp can be lit and dimmed. JP 11(1999)-111486 A discloses an example of a ballast for a discharge lamp that is supplied with a phase-controlled AC voltage to light and dim a fluorescent lamp. The ballast circuit of JP 11-111486 A includes a detection portion for detecting the conducting period of a phase-controlled AC voltage input and changes the brightness of the fluorescent lamp according to the detected conducting period.
In the above conventional ballast, when the fluorescent lamp is off, only a power smoothing capacitor in the ballast circuit is connected equivalently to a dimmer. Thus, the load characteristics become capacitive, causing malfunction of the dimmer. This makes the output waveform of the dimmer unstable, i.e., the waveform is different from a phase-controlled voltage waveform, as indicated by an example shown in FIGS. 6A and 6B. Specifically, when the fluorescent lamp having the waveform in FIG. 6A is dimmed increasingly during operation to be turned off for a while, it provides the waveform in FIG. 6B. As a result, the precise conducting period of a phase-controlled AC voltage cannot be detected. Therefore, when the fluorescent lamp in its non-operating state is started by adjusting the dimmer, it is impossible to restart the lamp according to the conducting period. Moreover, the ballast circuit malfunctions, causing problems such as flickering of the fluorescent lamp.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a ballast for a discharge lamp that is supplied with a phase-controlled AC voltage to light and dim a fluorescent lamp, the ballast being capable of detecting the conducting period of the phase-controlled AC voltage even in the non-operating state of the fluorescent lamp, restarting the lamp according to the conducting period, and preventing malfunction of a ballast circuit, such as flickering of the lamp.
To solve the above problems, a ballast for discharge lamp of the present invention includes a discharge lamp, an AC/DC conversion portion, a dimming control portion, and a DC/AC conversion portion. The AC/DC conversion portion converts a phase-controlled input AC voltage to a DC voltage. The dimming control portion calculates a dimming control signal from the phase-controlled input AC voltage. The DC/AC conversion portion converts an output voltage of the AC/DC conversion portion to a high frequency voltage to be applied to the discharge lamp and lights and dims the discharge lamp in response to the dimming control signal. The DC/AC conversion portion has a first operation mode that supplies the discharge lamp with a voltage for maintaining the lighting and a second operation mode that supplies the discharge lamp with a voltage lower than a starting voltage of the discharge lamp in its non-operating state.
This configuration can detect the conducting period of a phase-controlled AC voltage even when the fluorescent lamp is off, allowing the lamp to be restarted according to the conducting period. Also, the fluorescent lamp is supplied with a voltage lower than the starting voltage of the lamp in its non-operating state, preventing the lamp from flickering.
In the above configuration, the DC/AC conversion portion may switch the first and second operation modes in response to the dimming control signal.
It is preferable that the above configuration further includes a lamp characteristic detection portion for detecting the lamp characteristics of the discharge lamp, and that the DC/AC conversion portion switches from the first operation mode to the second operation mode in response to an output signal of the lamp characteristic detection portion. For example, the lamp characteristic detection portion detects the operating/non-operating state of the fluorescent lamp, and thus the first operation mode is switched to the second operation mode, which can prevent the ballast circuit failure. In this configuration, the lamp characteristic detection portion may detect at least a factor selected from a lamp voltage, lamp current, lamp power, and optical output as the lamp characteristics.
In any one of the above configurations, the DC/AC conversion portion may perform lighting and dimming by changing a driving frequency. In this case, f2 may be greater than f1, where f1 is a maximum driving frequency of the DC/AC conversion portion in the first operation mode and f2 is a driving frequency of the DC/AC conversion portion in the second operation mode.
A bulb-shaped fluorescent lamp may include a base and the ballast in any one of the above configurations, where the AC/DC conversion portion, the dimming control portion, the DC/AC conversion portion, and the discharge lamp are formed integrally.